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The Department of Homeland Security says it's working to identify who — or what — is behind an increasing number of attempted cyber attacks on U.S. election databases ahead of next month's midterms.

"We are aware of a growing volume of cyber activity targeting election infrastructure in 2018," the department's Cyber Mission Center said in an intelligence assessment issued last week and obtained by NBC News. "Numerous actors are regularly targeting election infrastructure, likely for different purposes, including to cause disruptive effects, steal sensitive data, and undermine confidence in the election."

The assessment said the federal government does not know who is behind the attacks, but it said all potential intrusions were either prevented or mitigated.